Internet Explorer 8 is the first version of IE to pass the Acid2 test, and the last of the major browsers to do so, although it scores 24/100 on the Acid3 Test. According to Microsoft, security, ease of use, and improvements in RSS, CSS, and Ajax support were its priorities for IE8.[4][5]

Internet Explorer 8 is the last version of Internet Explorer to run on Windows Server 2003 and Windows XP; the following version, Internet Explorer 9, works only on Windows Vista and later.[6][7] Support for Internet Explorer 8 is bound to the lifecycle of the Windows version it is installed on as it is considered an OS component, thus it is unsupported on Windows XP due to its end of extended support. Effective January 12, 2016, Internet Explorer 8 is no longer supported on any client or server version of Windows, due to new policies specifying that only the newest version of IE available for a supported version of Windows will be supported.[8][9] However several Windows Embedded versions will remain supported until their respective EOL, unless otherwise specified.[10]

IE8 development started in at least March 2006.[11] In February 2008, Microsoft sent out private invitations for IE8 Beta 1,[12] and on March 5, 2008, released Beta 1 to the general public,[13] although with a focus on web developers.[14] The release launched with a Windows Internet Explorer 8 Readiness Toolkit website promoting IE8 white papers, related software tools, and new features in addition to download links to the Beta.[13][15]Microsoft Developer Network (MSDN) added new sections detailing new IE8 technology.[13][16] Major press focused on a controversy about Version Targeting, and two new features then called WebSlice and Activities. The readiness toolkit was promoted as something "developers can exploit to make Internet Explorer 8 'light up'."[13]

On August 27, 2008, Microsoft made IE8 Beta 2 generally available.[17]PC World noted various Beta 2 features such as InPrivate mode, tab isolation and color-coding, and improved standards and compatibility compared to Internet Explorer 7.[18] Two name changes included Activities to Accelerators, and the IE7 Phishing filter renamed Safety Filter in the first Beta to SmartScreen, both accompanied by incremental technical changes as well.[18] By August 2008, the new feature called InPrivate had taken the spotlight.[18]

The first non-beta version was released on March 19, 2009.

On January 5, 2009, a tool was provided by Microsoft to block the automatic install of Internet Explorer 8 via Windows Update.

As of May 20, 2009, Windows XPe (Embedded) was not a supported platform for Internet Explorer 8. There have been no announcements of planned support.

On May 27, 2010, Microsoft released Internet Explorer 8 optimized for Bing and MSN. The file name is BOIE8_ENUS_XP.exe.[citation needed]

Language support (localization) was not complete on release. IE8 was released with 25 languages.[24] This grew up to 63 for Vista 32-bit in June 2009.[26] Support for additional languages can come pre-installed based on the OS, or downloaded and installed via Multilingual User Interface (MUI) packages.[25]

Accelerators are a form of selection-based search which allow a user to invoke an online service from any other page using only the mouse.[36] Actions such as selecting the text or other objects will give users access to the usable Accelerator services (such as blogging with the selected text, or viewing a map of a selected geographical location), which can then be invoked with the selected object. According to Microsoft, Accelerators eliminate the need to copy and paste content between web pages.[34] IE8 specifies an XML-based encoding which allows a web application or web service to be invoked as an Accelerator service. How the service will be invoked and for what categories of content it will show up are specified in the XML file.[37] Similarities have been drawn between Accelerators and the controversial Smart tags feature experimented with in the IE 6 Beta but withdrawn after criticism (though later included in MS Office).[14]

The address bar features domain highlighting for added security so that the top-level domain is shown in black whereas the other parts of the URL are grayed out. Domain highlighting cannot be turned off by users or web sites. Other features of the address bar include support for pasting multi-line URLs and an improved model for inserting the selection caret, and selecting words, or entire URLs in the Address bar. The inline autocomplete feature has been dropped from Internet Explorer 8, leading to criticism by beta users.[38]

If a website or add-on causes a tab to crash in Internet Explorer 8, only that tab is affected. The browser itself remains stable and other tabs remain unaffected, thereby minimizing any disruption to the browsing experience. If a tab unexpectedly closes or crashes it is automatically reloaded with the same content as before the crash.

Internet Explorer 8 now has replaced the Find... dialog box with an inline Find toolbar which can be activated by pressing CTRL+F or from search box drop-down menu but the F3 (or any other kind of a keyboard-driven) NextFind command has gone. Internet Explorer 8 highlights all instances of found words while allowing the user to continue the navigation normally.[40]

InPrivate Browsing in Internet Explorer 8 helps prevent one's browsing history, temporary Internet files, form data, cookies, and usernames and passwords from being retained by the browser, leaving no easily accessible evidence of browsing or search history. InPrivate Filtering provides users an added level of control and choice about the information that third party websites can use to track browsing activity. InPrivate Subscriptions allow you to augment the capability of InPrivate Blocking by subscribing to lists of websites to block or allow.

As with other private browsing modes there are ways that information about a browsing session can be recovered.

Internet Explorer 8 includes performance improvements across the HTML parser, CSS engine, mark-up tree manipulation as well as the JScript runtime and the associated garbage collector. Memory leaks due to inconsistent handling of circular references between JScript objects and DOM objects were corrected.[51] For better security and stability, IE8 uses the Loosely Coupled Internet Explorer (LCIE) architecture and runs the browser frame and tabs in separate processes. LCIE prevents glitches and hangs from bringing down the entire browser and leads to higher performance and scalability. Permissions for ActiveX controls have been made more flexible – instead of enabling or disabling them globally, they can now be allowed on a per-site basis.[52]

SmartScreen Filter extended Internet Explorer 7's phishing filter to include protection from socially engineered malware. Every website and download is checked against a local list of popular legitimate websites; if the site is not listed, the entire address is sent to Microsoft for further checks.[53] If it has been labeled as an impostor or harmful, Internet Explorer 8 will show a screen prompting that the site is reported harmful and shouldn't be visited. From there the user can either visit his or her homepage, visit the previous site, or continue to the unsafe page.[54] If a user attempts to download a file from a location reported harmful, then the download is cancelled. The effectiveness of SmartScreen filtering has been reported to be superior to socially engineered malware protection in other browsers.[55]

This feature is described by Microsoft as a tool to suggest websites, which is done by the browser sending information to Microsoft over a secure connection, which keeps the information and a per-session, uniquely generated identifier for a short time.[56] The Suggested Sites feature is turned off by default and is disabled when the user is browsing with InPrivate enabled or visiting SSL-secured, intranet, IP address, or IDN address sites. Information that could be personally identifiable, such as the user's IP address and browser information is sent to Microsoft as an artifact of the HTTPS protocol. Microsoft has stated that they do not store this information.

The functionality was defended by Microsoft after itworld.com's Gregg Keizer described it as a "phone home" feature.[57]

Web Slices are snippets of the entire page to which a user can subscribe.[36] Web Slices will be kept updated by the browser automatically, and can be viewed directly from the Favorites bar, complete with graphics and visuals. Developers can mark parts of the pages as Web Slices, using the hAtom and hSlicemicroformat. Web Slices have been compared to Active Desktop, introduced in Internet Explorer 4 in 1997.[58]

The information bar lets users allow an ActiveX control to run on all Web sites or only the current one. Users can easily make changes to this behavior through the Manage Add-ons dialog box. For each ActiveX control, there's a list of sites where it has been approved by the user.

DOM, which brings it in line with implementations in other browsers. Attributes and properties in DOM objects are now handled differently, and the behavior of the getAttribute, setAttribute and removeAttribute modifiers have been changed to match the behavior of other browsers.[37]

IE8 passes the Acid2 test,[73][74] but fails the Acid3 test with a score of 20/100. During its development, Microsoft developed over 7,000 tests for CSS level 2 compliance, which were submitted to the W3C for inclusion in their test suite.[75]

Internet Explorer 8 was promoted by Microsoft as having stricter adherence to W3C described web standards than Internet Explorer 7. As a result, as in every IE version before it, some percentage of web pages coded to the behavior of the older versions would break in IE8. This would have been a repetition of the situation with IE7 which, while having fixed bugs from IE6, broke pages that used the IE6-specific hacks to work around its non-compliance. This was especially a problem for offline HTML documents, which may not be updatable (e.g. stored on a read-only medium, such as a CD-ROM or DVD-ROM).

To avoid this situation, IE8 implements a form of Version Targeting whereby a page could be authored to a specific version of a browser using the X-UA-Compatible declaration either as a meta element or in the HTTP headers.[76]

In order to maintain backwards compatibility, sites can opt-into IE7-like handling of content by inserting a specially created meta element into the web page that triggers the "Compatibility mode" in the browser, using:[68]

Five weeks after the release of IE 8 Beta 2 in August 2008, Beta 1's market share had grown from 0.05 percent to 0.61 percent, according to Net Applications.[79] On July 2009, just under four months after the final release, the market share jumped to 13 percent.[80]

As of September 2013[update], estimates of IE8's global market share ranged from 6.93% to 8.73%.[81][82][83]

According to Net Applications web analytics for October, 2014, Internet Explorer 8 accounts for 17.31 % of web traffic, now overtaken by IE11 in usage.

In a March 19, 2009 review, Benny Har-Even of IT PRO offered some praise of Internet Explorer 8, noting its reliability and good features and concluding that it was "certainly the best version of Internet Explorer in a long time", but also that "there's not yet anything here to make Firefox users want to jump ship".[84] He offered praise to Microsoft for paying attention to their competition and producing "a better featured, faster and more reliable browsing experience for the masses" and suggested that as Microsoft continues to improve the product, it would become "harder to persuade the unconverted to switch away from IE."[84]

The next month, on April 2, Mark Joseph Edwards wrote in the newsletter Windows Secrets that the new edition of Internet Explorer had greatly improved security, speed and compatibility, but opined that it still lagged behind competitors in all three areas.[85] Edwards noted that at the time Internet Explorer 8 was still underperforming relative to other browsers in speed and was not as successful in displaying webpages as they were intended to display as such browsers as Firefox and Opera. In terms of security, he wrote that its "continued reliance on ActiveX makes the browser vulnerable in its very foundation."[85] For these reasons, he suggested that Firefox remained a better alternative to Internet Explorer 8, even though it was a "much better browser than IE 7."[85]